Force and pressure

A liquid presses on the sides and on the bottom of the container in which it rests. It exerts a force both sideways and downwards'. The sides of a cardboard carton of milk will often curve outwards, and this is caused by the sideways force exerted by the milk. Wooden barrels and water tanks are usually strengthened with metal bands: they are reinforced to resist the sideways force exerted by the liquid in these containers.

A liquid also exerts a force upwards on any object which is placed in it. If you push a rubber ball or a plastic boat under the surface of the water in a container, you will feel the upward force against the bottom of these objects. A liquid exerts a force in every direction.

When a liquid presses against the container in which it rests, we say that it exerts a force. Force, however, is not the same as pressure.

A block of metal of mass 10 kg resting on a flat surface such as a table exerts a downward force of about 93 Newton. The downward force exerted by an object is the same as its weight and we measure weight in force units called Newton. But the object does not exert a pressure of 98 N. The pressure which is exerted depends on the area of contact between the object and the table. Pressure is measured by the force divided by the area of the surface on which it acts. Let us suppose that the area of one of the surfaces of an oblong block of steel weighing 200 N is 200 cm (0 • 02 m). If the object is placed on a table-top so that this surface is in contact with the Surface of the table, the pressure between the object and the table will be 200 N/200 cm², which is 10,000 N/m². If the other surface of the object measures 100 cm² (0-01 m²), and if this makes contact with the table-top, the pressure between the object and the table will be 200 N/100 cm², which equals 20,000 N/m. The force exerted by the object is the same in each case, but the pressure varies. Less pressure is exerted when the area of contact is large, and more pressure is exerted when the area of contact is small.